[IRCA] Highly Technical (and possibly Vastly Boring): SRF 59 Ultralight radio mystery continues

["Steve Ratzlaff" <steveratz@xxxxxxxx>]

I received my SRF 59 Ultralight radio yesterday, thanks to the great 
generosity of Gary Debock. I spent some time initially trying to compare its 
sensitivity to my ancient Sony 2010. I do not live close to any strong BCB 
stations--1450 KLBM is my closest one, about 20 miles away in La Grande, OR. 
The next strongest one is 610 in Tri-Cities, WA, about 100 air miles away. 
Starting about 1:30-2:00p.m. I began comparing all the stations the SRF 59 
could receive, but by 2 the eastern skip was already coming in, obscuring 
results.
But the 2010 was clearly hearing more signals, or the same signals with less 
noise (in Wide, non-sync mode).
Gary's tests indicated my Ultralight radio needed the sensitivity-peaking 
adjustment. That might be something I would attempt at some later time.
I made a 4" ferrite coupling coil which I attached to my external antenna 
switching arrangement. Simply placing the ferrite rod either in line with 
the top or in line with the left side made a huge improvement--I could 
easily hear lots of signals, with no hints of overloading either. (I have a 
1600' E/W and a 400' N/S longwire to choose from.)
This morning I checked for TPs--while I did hear some, they were barely 
heard in SSB mode with the Icom R-75 tabletop radio--nowhere close to being 
strong enough to be received in AM mode on the SRF 59. I've only rarely had 
TP signals strong enough to be decently heard on the tabletop--it's doubtful 
I could ever say I'd heard a TP with the SRF 59 at this location.

Today I decided I'd try Gary's instructions for opening up the little pocket 
radio and at least examine what it looked like inside. As when I initially 
tried to remove the battery cover to install the single AA cell, my radio is 
extremely difficult to get that battery cover off, and it wasn't any easier 
the second time around! But nothing broke and after removing the battery and 
the two screws, Gary's procedure for separating the case halves eventually 
produced the desired result--the bottom half finally came free. At this 
point the PCB chassis is still in the top case half, and everything is still 
operable if you reinstall the battery.
As Gary's notes say, be sure to carefully note the position of the orange 
tuning tab, with the tuning as far to the bottom of the tuning range as the 
dial will go. You definitely will need to know where that tab is positioned 
to put everything back together again in the same tuning range as before.
The PCB chassis is loose at the bottom, and can be raised slightly, but as 
Gary's notes indicate, the top is held to the top case half with a blob of 
hotmelt-type glue. By raising the bottom of the PCB chassis, the PCB will be 
partially removed, leaving the glue joint clearly exposed. You can simply 
continue pulling the PCB up until the glue separates. That's when the orange 
tuning strip will become separated from the toothed wheel attached to the 
tuning capacitor on the other side of the PCB. And the tuning strip won't 
fall out since the top case half is still below the PCB.
Turning the PCB over, small metal transformer can with the red adjustment 
slug is readily seen, at the top of the PCB next to the small ferrite rod 
antenna. As Gary's notes say, that's the transformer that adjusts the 
alignment for best sensitivity. Note there's a second metal transformer can, 
but it has a black adjustment slug--do not touch that! (That's for the FM 
stereo multiplex adjustment.)
I'm blessed with a modest home lab with some test equipment including an RF 
signal generator as well as a spectrum analyzer. I had already turned both 
on to warm up when I'd decided to open up the SRF 59. Setting the generator 
to 800 kHz, with AM modulation turned on, and positioning the output lead 
near the ferrite antenna, I eventually was able to tune the signal in, on 
the radio, and adjusted the generator level for a weak signal. Using a 
nonmetallic tuning tool, I then adjusted the red slug to see what happened. 
Turning it definitely peaks the signal, and it only has one fairly sharp 
peak. But my radio was already peaked for best sensitivity, it turned out.
I then used the spectrum analyzer to try to find the local oscillator 
signal. This is where the mystery part begins. This radio definitely does 
not have the traditional 455 kHz IF, nor any other traditional IF that I 
could find. I was able to see two discrete frequencies that moved back and 
forth as the tuning capacitor was rotated. Eventually I found that the upper 
frequency was merely the second harmonic of the lower frequency. And the 
lower frequency is approximately 57 kHz above the RF tuned frequency!
The tuning range of my radio, as found with the signal generator, is about 
520-1774 kHz. The corresponding supposed-local oscillator therefore tunes 
from 577-1831 kHz. No other signals were found coming from the radio, at 
least radiated to the pickup loop of the spectrum analyzer.
If the radio were one of the fancy modern I-Q direct conversion types, then 
the local oscillator should be running directly at some multiple of the 
received frequency (2 or 4 times, for example). Clearly that's not the case, 
at least from what I'm observing.
Perhaps the RF is directly converted to the approximate 57 kHz and the 
30-pin IC1 integrated circuit uses DSP processing to derive the final audio 
output? And something similar may also be happening when in the FM mode. As 
Gary has noted, there is another integrated circuit, IC2, which is probably 
the headphone driver. There are a number of surface mount resistors and 
capacitors on the bottom side of the board, but no active devices, as far as 
I can tell. There's a small two-lead device associated with the tuning 
capacitor, that I suspect may be a SAW resonator for the FM local 
oscillator; it also has a long PC trace connecting it to the tuning 
cap--that may be an inductor.
All in all, this is a very interesting circuit board, and quite mysterious!
Who will be the first to unlock all the mysteries of this little radio?

As a postscript, I didn't find it particularly difficult to reassemble the 
radio into a properly functioning unit, with all parts back in place. :) I'm 
by no means proficient at working with these tiny import radios, and perhaps 
it's just dumb luck on my part.
My procedure was similar to Gary's.
Looking at the bottom of the PCB, with the tuning dial to the left, rotate 
the tuning dial fully clockwise, to its low-frequency position. Position the 
orange tuning strip in the same place as initially noted, then place the top 
case half with the tuning strip in it, face down. Carefully insert the top 
PCB section headphone jack into its slot in the case half,(with the bottom 
side of the PCB that has the integrated circuits facing you). At this stage 
the PCB is only partially into the case half, buts the headphone jack is 
holding it in place. Slowly lower the PCB into the case half, making sure 
the two metal battery spring wires are partially inserted into their 
corresponding slots in the case half. The orange tuning strip should be 
extending straight out to the left, and probably on its side, not meshed 
with the tuning cap gear. Using tweezers, turn the strip so its gearing 
meshes with the tuning cap gear. Then use the tweezers to push the end into 
the case half so it's now inside the case.
You can then finish pushing the PCB onto the case half, again making sure 
both battery wires are in their slots in the case half. The PCB will now be 
in place, and the two holes where the screws go should be mating with the 
holes in the PCB.
Rotate the tuning dial and make sure the tuning tab moves and is not 
binding. (Maybe I just got lucky, but this procedure worked the first time 
for me.)
You can now carefully put the bottom case half in place, starting from the 
top, and snap it in place. Insert the two screws and tighten them. If your 
battery cover came off, put it back in place; install a battery, and your 
radio should be the same as when you started. If you also did Gary's 
sensitivity adjustment, and your radio needed peaking, then your radio 
should be more sensitive now.

We now need someone else besides Gary and me to be brave and take their 
beloved SRF 59 apart and see how the process works for them, reassembling 
it! How about that self-described Technical Director, with the initials 
"PVZ", who lives down there in Manasota Keys, Florida----I bet he's up to 
the task, having many years experience with the vastly more intricate boat 
anchor radios like his beloved R-390 and R-390A's. :))

Best regards,
Steve
NE Oregon 

_______________________________________________
IRCA mailing list
IRCA@xxxxxxxxxxxxxxxx
http://montreal.kotalampi.com/mailman/listinfo/irca

Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers

For more information: http://www.ircaonline.org

To Post a message: irca@xxxxxxxxxxxxxxxx